This disclosure relates to marine seismic data acquisition, and more particularly to technology for positioning a front portion of an array of streamers.
Seismic exploration involves surveying subterranean geological formations for hydrocarbon deposits. A seismic survey typically involves deploying seismic source(s) and seismic sensors at predetermined locations. The sources generate seismic waves, which propagate into the geological formations creating pressure changes and vibrations along their way. Changes in elastic properties of the geological formation scatter the seismic waves, changing their direction of propagation and other properties. Part of the energy emitted by the sources reaches the seismic sensors. Some seismic sensors are sensitive to pressure Changes (hydrophones), others to particle motion (e.g., geophones), and industrial surveys may deploy only one type of sensors or both. In response to the detected seismic events, the sensors generate electrical signals to produce seismic data. Analysis of the seismic data can then indicate the presence or absence of probable locations of hydrocarbon deposits.
Some surveys are known as “marine” surveys because they are conducted in marine environments. However, “marine” surveys may be conducted not only in saltwater environments, but also in fresh and brackish waters. In one type of marine survey, called a “towed-array” survey, an array of seismic sensors and source(s) is towed behind a survey vessel. The seismic sensors are contained within or on streamers towed by the survey vessel.
In cases where multiple streamers are used for seismic data acquisition, deflector devices (e.g., doors and/or mono-wings, etc.) are often used to maintain streamer separation. Lead-in devices are then used to connect the vessel to the deflectors. In order to maintain the streamer depth at the front of the array, floats or paravanes are used to get sufficient buoyancy of the front, and hence avoid the front to dive. In order to determine the position of the streamers, GPS units have often been used on the floats or on special dedicated front end positioning floats. These floats are cumbersome and difficult to maintain as they are sitting on the front of the streamers. Furthermore, they require increased mobilization time and hence make the operation less efficient.
Another drawback of having the GPS on the float is that it needs power and such power comes from the lead-in or the streamer as battery power is not sufficient for powering the GPS and its associated communication radio. Since the deflectors are large, and ropes and chains used to connect the paravane and wings are exposed to large forces, it is difficult to maintain electrical wiring from the streamers to the floats. In addition, extra equipment is needed to connect a front end GPS.
U.S. Pat. No. 7,315,274 to Kongsberg Seatex AS discloses a method for determining the position of marine vessels and objects through the use of an interrogator and transponder. The '274 patent describes the use of the transponder for including identity tags into the signal to be reflected to the interrogator. Such method, however, still requires power on the marine objects on which the transponders are positioned.